Blender 2.83 - water flow
Scroll down to Cache inside the physics settings. Switch the Cache Type to modular. Replay (the default cache type in 2.83+) will allow the simulation to run when you are playing the animation, modular actually bakes it onto the disk.  You can also switch it to Final to bake everything at once, but note that final bake.
A lot of people have this problem when they watch tutorials done in a previous release of blender and don't know what to do because some things changed. Reading the documentation should clear everything up. It is written very clearly. https://docs.blender.org/manuel/en/latest